DCHECK that hostname is not empty in MarkAlternativeServiceBroken.

net/http/http_server_properties_impl.cc

Index: net/http/http_server_properties_impl.cc
diff --git a/net/http/http_server_properties_impl.cc b/net/http/http_server_properties_impl.cc
index 2484371ada1b03c38d0861236a7d7b91bee86079..fef2058fab552d64861f6d4add518f3a76c88ab9 100644
--- a/net/http/http_server_properties_impl.cc
+++ b/net/http/http_server_properties_impl.cc
@@ -315,6 +315,8 @@ void HttpServerPropertiesImpl::SetAlternativeService(
 
 void HttpServerPropertiesImpl::MarkAlternativeServiceBroken(
     const AlternativeService& alternative_service) {
+  // Empty host means use host of origin, callers are supposed to substitute.
+  DCHECK(!alternative_service.host.empty());
   if (alternative_service.protocol == UNINITIALIZED_ALTERNATE_PROTOCOL) {
     LOG(DFATAL) << "Trying to mark unknown alternate protocol broken.";
     return;